1. Field of the Invention
This invention relates to a platemaking apparatus for making printing plates for use in letterpress printing such as flexography, and in intaglio printing such as photogravure.
2. Description of the Related Art
Conventional platemaking apparatus of the type noted above include a laser engraving machine as described in U.S. Pat. No. 5,327,167, for example. This laser engraving machine makes letterpress printing plates by scanning a recording material with a laser beam emitted from a laser source to engrave the surface of the recording material. The machine includes a modulator for modulating the laser beam emitted from the laser source, a recording drum rotatable with the recording material mounted peripherally thereof, and a recording head movable in a direction parallel to the axis of the recording drum for irradiating the recording material mounted peripherally of the recording drum with the laser beam emitted from the laser source.
In such a platemaking apparatus for making printing plates, the main scanning speed of the laser beam, i.e. the rotating speed of the recording drum, is set to a value for obtaining a required maximum engraving depth, based on the power of the laser source and the sensitivity of the recording material. Areas shallower than the maximum engraving depth are engraved by reducing the power of the laser beam emitted to the recording material. A relatively large amount of energy is required for engraving the recording material with a laser beam. Thus, there is a drawback of consuming a relatively long time in the platemaking process.
Japanese Patent No. 3556204 discloses a printing block manufacturing method for creating relief by emitting a plurality of laser beams simultaneously to a recording material.
Further, Applicant herein has proposed a platemaking apparatus for engraving a recording material by irradiating the recording material at a first pixel pitch with a laser beam having a first beam diameter, and thereafter irradiating the recording material at a second pixel pitch different from the first pixel pitch with a laser beam having a second beam diameter different from the first beam diameter (Japanese Patent Applications Nos. 2004-286175 and 2004-357586). With this platemaking apparatus, the platemaking time may be shortened by using the laser beams efficiently.
The printing block manufacturing method described in Japanese Patent No. 3556204 noted above can create relief efficiently by emitting a plurality of laser beams simultaneously to a recording material. However, it is difficult to obtain precise engraving results since the laser beams are moved at a fixed pixel pitch. On the other hand, where a recording material is engraved by irradiating the recording material at a first pixel pitch with a laser beam having a first beam diameter, and thereafter irradiating the recording material at a second pixel pitch different from the first pixel pitch with a laser beam having a second beam diameter different from the first beam diameter, a precise engraving may be carried out efficiently, but the engraving requires two steps for its completion. Thus, an engraving process of enhanced efficiency is desired.